This invention relates to a method for machining the peripheries of out-of-round workpieces and to a lathe for carrying out this method. The invention is especially useful in the machining of out-of-round raw piston rings with gap sections not yet cut out.
Lathe-like apparatus incorporating a copying device have been employed for the peripheral machining of piston rings having out-of-round inner and outer circumferences. In such apparatus, the piston rings are axially clamped together to form a stack and mounted on a driven spindle for simultaneous machining of their inner and outer circumferences. The tools are attached to the ends of pivotal tool holders provided with a radial adjustment that is controllable by a copying cam rotating in synchronism with the work spindle. In particular, an adjustable dual lever system may be used for controlling the tool for out-of-round machining of piston rings, such apparatus having been operated satisfactorily for many years.
A disadvantage of the prior art machining apparatus is that the maximum number of revolutions of the spindle depends upon the out-of-round shape of the piston ring. Specifically, in the area of the piston ring where the gap is cut after machining and where there is a relatively slight curvature, the copying lever system does not provide a reliable, accurate transfer of the out-of-round contour of the workspindle. This occurs because, with an increase in the number of revolutions in synchronism with the copying cam, the copying roller is lifted off the cam or the mass inertia of the copying lever system is insufficient to assure accurate transfer of the out-of-round contour.
Recently, cutting tools have been developed which permit very high cutting speed thereby permitting an increase in the output of the machining apparatus. This increase in the relative cutting speed between the tool and workpiece requires a simultaneous reduction of the cutting depth or advance in order to reduce mechanical and thermal stresses on the workpiece during the cutting process. Accordingly, it is now possible to reduce to a minumum the undesirable internal stresses which occur in the workpiece structure as a result of the machining process.
It is an object of the present invention to provide a method, and a lathe operating according to this method, which provides optimum cutting conditions during machining of the circumferences of out-of-round workpieces such as piston rings.